1. Field of the Invention
This invention relates to seismic prospecting and to the generation of seismic waves by gas exploders. More particularly, this invention relates to an apparatus and method for imparting successive seismic impulses at the earth's surface at a high repetition rate utilizing a gas exploder.
2. Description of the Prior Art
Rapidly cycled gas exploder seismic sources may be used to generate coded sequences of seismic impulses which are highly important in the development of computerized data acquisition systems. One limiting feature in any such cyclic operation is the speed with which the recoil and recovery of the movable piston of the exploder may be accomplished. This invention is primarily directed toward this problem.
As described in co-pending commonly assigned application Ser. No. 526,842 entitled A HIGH SPEED SEISMIC GAS EXPLODER AND METHOD OF OPERATION filed Nov. 25, 1974, in the name of Daniel P. Hearn, now U.S. Pat. No. 3,995,713 a seismic gas exploder for injecting seismic signals into the earth is provided with a high speed hydraulic catching apparatus. With the aid of this apparatus, the exploder rebounds essentially unrestrained after being fired at the surface of the earth and thereafter falls freely from its maximum rebound height to within a few inches of the ground. The exploder is then lowered slowly to its starting position under hydraulic damping. According to one alternate feature of this prior art invention, the upward travel of a piston connected to the gas exploder is adapted to pressurize an enclosed volume of air which is then available to aid gravity in accelerating the downward movement of the exploder until a predetermined intermediate position is reached.
The present invention makes use of a compressed air energy source such as described above in a novel recoil and recovery system for the movable piston of a seismic gas exploder. It is an essential feature of the system to be described, however, that the exploder housing does not itself leave the earth, as in the above-referenced application. This is accomplished by taking advantage of the so-called "free base" construction more particularly described in co-pending Airhart patent application Ser. No. 650,370, filed Jan. 19, 1976, now U.S. Pat. No. 4,007,803, and by eliminating any spring biasing means directly interconnecting the housing and the movable piston.